Hydroforming is now widely employed for forming strong and light weight metal members for vehicle chassis and the like. In hydroforming, a tubular metal blank is placed into a mold and the blank is then expanded to conform to the mold by filling the blank with pressurized fluid, typically water.
As a follow on step to the hydroforming process, before depressurizing the formed member and removing it from the mold, it is known to punch any needed apertures into the member with punches. These punches are typically operated via hydraulic pistons and operate much like conventional punch and dies, except the pressure of the fluid in the hydroformed member removes the need for a die, as the pressure holds the wall of the member against the movement of the punch.
To avoid an additional manufacturing step, the slugs formed by the punching of the apertures are often held captive within the hydroformed member by a feature on one side of the punch which ensures that a portion of one side of the slug remains attached to the member. The slug is folded back about this portion, into the interior of the member, by the movement of the punch during the punch operation, and is held captive by this portion without obscuring the opening of the aperture.
However, when it is desired to punch relatively large (in comparison to the diameter of the hydroformed member) apertures, it can be difficult or impossible to form such apertures as the slug may abut a portion of the interior of the member during the punching operation.
Even in circumstances wherein the large aperture can be punched, it may be impossible to form a captive slug, as the slug may abut a portion of the interior of the member as it is folded, preventing the slug from being folded out of the way and thus at least partially obscuring a portion of the opening of the aperture.
Accordingly, to punch such relatively large apertures, it is known to use a stepped punch to form a desired aperture with two smaller slugs. If the slugs are to be captive, each slug is held captive about a different portion of the aperture and is folded back away from the other slug.
However, conventional stepped punch systems require very high punch pressures and calibration pressures (the pressure of the fluid in the hydroformed member) to operate and thus the tooling and rams for such systems are expensive and/or the cycle times for the forming and punching operation can be longer than would otherwise be the case. More significantly, stepped punches can prevent the feeding of an additional amount of blank into the mold during hydroforming, which is an often desired operation.